The weight of an electron is incredibly small, on the order of 10^-30 kg. To compare this to the difference in mass between a fully charged and uncharged 400 Wh battery, which is about 1.6 x 10^-17 kg, the weight of an electron is about 6 orders of magnitude smaller. In other words, the difference in mass between a fully charged and uncharged 400 Wh battery is about 1,600,000,000,000,000 times greater than the weight of an electron. So the difference in mass between a fully charged and uncharged battery is much, much larger than the weight of an electron. It's important to keep in mind that the difference in mass between a fully charged and uncharged battery is still incredibly small, on the order of picograms, so it would be impossible to detect in practice.